1. Field of Invention
The present invention relates to a type of horizontad injector for an oxidation furnace. More particularly, the present invention relate, to an oxidation furnace having a horizontal injector whose hydrogen/nitrogen gas inlet is changed from a side position to a back position to avoid cracking the injector.
2. Description of Related Art
In the fabrication of semiconductor devices, the supply of heat has become an indispensable part of most processes. The most commonly used thermal diffusion equipment, for example, a thermal diffusion furnace, can be classified according to whether it is a horizontal or a vertical type. The horizontal type is the earliest, and is still widely employed in most processes.
Another type of commonly used thermal diffusion equipment is the thermal oxidation furnace. The construction and structure of a thermal oxidation furnace is very similar to a thermal diffusion furnace. A thermal oxidation furnace also can be classified as a horizontal or a vertical type, and both types operate at atmospheric pressure. Beside thermal oxidation furnaces that operate mainly in atmospheric pressure, specially designed high atmospheric pressure oxidation furnaces and plasma-operated oxidation furnaces are available too. However, these types of furnaces are not so commonly used in the semiconductor industry. Therefore, oxidation is still normally carried out using either a horizontal or a vertical atmospheric pressure oxidation furnace.
Fundamentally, oxidation operations can be subdivided into dry oxidation or wet oxidation. Equipment for carrying out a dry oxidation is relatively simple. The only criterion is to allow suitable amounts of passivation gases or nitrogen gas to pass into a heated furnace chamber (roughly at a temperature of above 900.degree. C.). A layer of oxide will begin to grow on the surface of the wafers stationed inside the reaction chamber. On the other hand, equipment for carrying out a wet oxidation reaction is slightly more complicated. As a rule, moisture is not directly used as an agent in the wet oxidation reaction. Rather, gaseous hydrogen and gaseous oxygen are passed into a heated chamber (roughly at a temperature of above 600.degree. C.) to form moisture, and the moisture is indirectly used in the oxidative reaction. Chemical formula (a) below illustrates the chemical reaction involved. As shown in formula (a), water, which is a product of the chemical reaction between hydrogen and oxygen, has an exceptionally high purity. Because of this, the silicon dioxide (SiO.sub.2) layer grown on a silicon wafer using a wet oxidation method has better electrical properties. However, the application of moisture generated through reaction (a) for carrying out necessary oxidation is not that simple. This is because gaseous hydrogen is a combustible gas. If gaseous hydrogen is not properly consumed, a pipe explosion may occur. Therefore, the flow of gaseous hydrogen must be handled carefully. In general, a higher furnace temperature is able to prevent the accumulation of unreacted hydrogen, thereby avoiding a hydrogen "explosion" inside the reaction chamber. ##EQU1##
The basic structure of a conventional horizontal oxidation furnace is the same as that of a thermal diffusion furnace. Their main difference lies in the design of inlets for introducing gaseous reactant. FIG. 1 is a diagram showing the injector structure of a conventional horizontal oxidation furnace. Since the reaction as shown in formula (a) can easily occur at a high temperature and an oxygen-filled atmosphere, a quartz injector 10 is used. Gaseous hydrogen (H.sub.2) and gaseous oxygen (O.sub.2) enter the quartz injector 10 through two gaseous inlets 11b and 12b respectively, and are injected into a pipe furnace 15. Hydrogen passes into the inner pipeline through the gas inlet 11b and out of the inner pipeline through a gas outlet 11c into the pipe furnace 15. Similarly, gaseous oxygen (O.sub.2) passes into the outer pipeline through the gas inlet 12b and out of the outer pipeline through a gas outlet 12c into the pipe furnace 15.
The conventional injector 10 includes an inner tube 11 and an outer tube 12, wherein the outer tube 12 wraps around the inner tube 11 but exposes a portion of the inner tube 11 outside the outer tube 12 area. The inner tube 11 and the outer tube 12 have branch tubes 11a and 12a and gas outlets 11c and 12c, respectively. Furthermore, the branch tube 11a has an inner tube inlet 11b, and the branch tube 12a has an outer tube inlet 12b. The branch tube 11a is perpendicular to the inner tube 11, and the branch tube 12a is perpendicular to the outer tube 12.
Before starting a wet oxidation reaction, oxygen is first passed into the pipe furnace 15 until the whole furnace 15 is oxygen-filled. Next, a suitable amount of hydrogen is injected into the furnace 15 through the inner tube inlet 11b and inner tube outlet 11c of the injector 10. Because the furnace already, has a sufficient amount of oxygen inside, the reaction indicated by formula (a) can be carried out. To avoid an explosion caused by a shortage of oxygen or the accumulation of hydrogen, the amount of oxygen flowing into the furnace 15 must be maintained at slightly more than half the amount of hydrogen going into the furnace. In other words, since the rate of consumption of hydrogen and oxygen is in a molar ratio of 1:1/2, a slight excess of oxygen must be maintained inside the furnace throughout the wet oxidation operation. Similarly, when wet oxidation is finished, although the inflow of hydrogen has stopped, oxygen must still be supplied for quite awhile so that all the remaining hydrogen inside the furnace can react with oxygen. Finally, nitrogen is passed into the furnace 15 via the inner tube 11 while the furnace 15 is allowed to cool down.
The above description shows that even when the wet oxidation process has finished and inflow of hydrogen has stopped, oxygen needs to be supplied for a certain period more so that all the remaining hydrogen inside the furnace has reacted. However, the continuous supply of oxygen to the furnace has the adverse effect of flowing back through the inner tube 11 into the inner branch tube 11a. Hence, an explosive force caused by the oxygen/hydrogen mixture will be transmitted to the junction area 13 where the inner tube and the branch tube 11a are joined together. Consequently, the junction area 13 can be easily fractured.
Furthermore, as shown in FIG. 1, the conventional injection 10 has a side hydrogen/nitrogen (H.sub.2 /N.sub.2) gas inlet, and the fusing of a side inlet to the main tube can add a lot of internal stress at the junction. Therefore, the junction area of the injection 10 is rather weak and can easily crack when subjected to a minor explosive force.
In summary, the conventional injector structure has the following defects:
(1) Because the branch tube is connected on one side of the inner tube, most of the explosive force is concentrated there. Therefore, the junction area can easily break.
(2) The hydrogen/nitrogen gas inlet of a conventional injector is attached to one side by fusion. Therefore, the fusion area has a lot of accumulated internal stress, which can easily be ruptured by slight explosive pressure.
In light of the foregoing, there is a need to produce a better injector design.